A solid catalyst component that includes magnesium, titanium, an electron donor compound, and a halogen as essential components has been used when polymerizing an olefin (e.g., propylene). Various methods have been proposed that polymerize or copolymerize an olefin in the presence of an olefin polymerization catalyst that includes the solid catalyst component, an organoaluminum compound, and an organosilicon compound.
For example, Patent Document 1 (JP-A-57-63310) and Patent Document 2 (JP-A-57-63311) disclose a method that polymerizes an olefin having 3 or more carbon atoms using a catalyst that includes a solid catalyst component, an organoaluminum compound, and an organosilicon compound having an Si—O—C linkage, the solid catalyst component including a magnesium compound, a titanium compound, and an electron donor. However, since the above method is not necessarily satisfactory in order to obtain a polymer having high stereoregularity in high yield, a further improvement has been desired.
Patent Document 3 (JP-A-63-3010) discloses a propylene polymerization catalyst and a propylene polymerization method, the propylene polymerization catalyst including a solid catalyst component, an organoaluminum compound, and an organosilicon compound, the solid catalyst component being obtained by heating a powder product obtained by bringing a dialkoxymagnesium, an aromatic dicarboxylic diester, an aromatic hydrocarbon compound, and a titanium halide compound. Patent Document 4 (JP-A-1-315406) discloses a propylene polymerization catalyst and a propylene polymerization method that utilizes the propylene polymerization catalyst, the propylene polymerization catalyst including a solid catalyst component, an organoaluminum compound, and an organosilicon compound, the solid catalyst component being prepared by bringing titanium tetrachloride into contact with a suspension that includes a diethoxymagnesium and an alkylbenzene, reacting phthalic dichloride with the mixture to obtain a solid product, and reacting the solid product with titanium tetrachloride in the presence of an alkylbenzene. The above techniques aim to achieve high activity that makes it possible to omit a deashing step that removes catalyst residues (e.g., chlorine and titanium) from the resulting polymer, improve the yield of the stereoregular polymer, and maintain the catalytic activity during polymerization. Good results are obtained by the above techniques. An olefin polymer produced using such a catalyst is melted, and molded (using molding machine or stretching machine) into a product used for a variety of applications (e.g., vehicle, household electric appliance, container, and film). However, since such an olefin polymer exhibits a melt flow rate insufficient for high-speed stretching and high-speed injection molding, various studies have been conducted in order to deal with the above problem.
The melt flow rate of an olefin polymer significantly varies depending on the molecular weight of the olefin polymer. For example, an olefin polymer having a low molecular weight has a high melt flow rate. Therefore, the molecular weight of an olefin polymer is normally reduced by adding hydrogen during polymerization in order to obtain an olefin polymer having a high melt flow rate. In this case, a large amount of hydrogen is normally added during polymerization.
However, the amount of hydrogen that can be added during polymerization is limited due to the pressure resistance of the polymerization reactor. Therefore, it is necessary to reduce the partial pressure of olefin gas subjected to polymerization in order to add a larger amount of hydrogen. In this case, the productivity inevitably decreases. Moreover, the stereoregularity of the resulting olefin polymer may deteriorate when a large amount of hydrogen is used. It is also uneconomical. Therefore, development of a catalyst has been desired that makes it possible to produce an olefin polymer having a high melt flow rate using a small amount of hydrogen (i.e., exhibits high activity with respect to hydrogen), and produce an olefin polymer having high stereoregularity in high yield.
For example, Patent Document 5 (JP-A-2004-107462) discloses a propylene polymerization catalyst and a propylene polymerization method, the propylene polymerization catalyst including a solid catalyst component, an organoaluminum compound, and an organosilicon compound, the solid catalyst component being obtained by bringing a dialkoxymagnesium, a titanium halide compound, a phthalic diester, and a malonic diester into contact with each other.
The propylene polymerization catalyst disclosed in Patent Document 5 exhibits good activity with respect to hydrogen as compared with a known catalyst that utilizes a phthalic diester. However, the propylene polymerization catalyst disclosed in Patent Document 5 does not necessarily exhibit catalytic performance that ensures both stereoregularity and hydrogen response (i.e., lacks a capability to produce a polymer having both high stereoregularity and a high melt flow rate). Therefore, a further improvement has been desired. Moreover, development of a catalyst that makes it possible to produce polymer particles having a higher bulk density has also been desired.